Sintering, which is a common method of manufacturing porous plastic materials, is a process wherein plastic particles or pellets are fused together by heat. It has been reported that the sinterability of a material (i.e., how well it sinters) is a function of its melt-flow index (MFI). See, e.g., U.S. Pat. No. 3,954,927 to Duling et al.
The MFI of a material reflects the degree to which it flows when subjected to a given weight at a given temperature. For example, ultra-high molecular weight polyethylene has no measurable melt-flow rate at 190° C. under a 21,600 gram load. See ASTM D1238-65T. In general, polyethylene particles that have a melt-flow index of 0 to 6 grams per 10 minutes at 190° C. under a 21,600 gram load sinter evenly to provide a final product with desirable mechanical properties and porosity. By contrast, it has been observed that polyethylene particles with high MFIs (i.e., greater than about 6 grams per 10 minutes under these conditions) simply melt during the heating and cooling cycle and form solid non-porous product, or only partially sintered and form weak or deformed product. This results in sintered porous products that are inconsistent in terms of their shape, mechanical properties, porosities and average pores sizes.
Sintered porous plastic materials with high MFI materials would provide advantages over conventional low MFI polymers. High MFI materials are easy to be compounded and many additives can be added to high MFI polymer materials without significant difficulty. However, low MFI materials are difficult to compound because of their poor flow properties in the extruder. The sintered porous plastics from high MFI materials will have new properties bring in by the additives. Compared with low MFI materials, high MFI materials generally are less expensive and will have cost advantage over low MFI polymer materials.
A process that allows the sintering of high MFI materials would provide a variety of advantages. Such advantages could include, for example, the ability to use relatively inexpensive raw materials, instead of the typically more expensive low MFI materials. Such a process could also reduce the cycle times necessary for the production of thick products. Until now, however, it has not been thought possible to sinter high MFI materials.